As a response and alternative to Cartesian dualism, behaviourism was the widely accepted interpretation of human mind activities for most of the twentieth-century. Ryle as an analytical behaviourism philosopher argued that mental terms represent the style of behaviours or dispositions, not the internal states (Ryle, 2009, pp. 1-13). In contrary, Fodor questioned if Ryle’s theory can address all human behaviours as there are many mental terms that their dispositional versions require some other mental terms. He appended that, even though, all mental conditions can be analysed to their dispositional forms, it only defines mental terms while, on the other hand, psychology looks for causal analysis of mental activities. Fodor accused Ryle of confusing between conceptual and causal theories about the mind.

Fodor challenged the behaviourist by making a distinction between dualism and mentalism, which he believed that their confusion was “the original sin of the Wittgensteinian tradition” (Fodor, 1975, p. 4). He posited that the mentalism can be expressed as the existence of casually effectual mental states while dualism requires a particular type of substance, in addition, to justifying mental activities. He argued that behaviourists tried to expel ‘the ghost from the machine’ by logically reducing behaviours to mental states. However, the choice between behaviourism and Cartesian dualism is not comprehensive and complete. In other words, it is not required to be a behaviourist to avoid being a dualist: what is known now as functionalism (Fodor, 1968, pp. 58-59).

He proposed that by ascribing mental states to organisms that can influence and control their environments the need for a nonphysical substance different from human brains is withdrawn. In addition, Fodor argued that knowing about the mental states of others does not necessarily imply the requirement of a logical relation between behavioural and mental terms. In fact, linguistics and cognitive theories are potent arguments against behaviorism, as they can postulate various mental events that are not determinable as observable behaviors nor need causal relations (Fodor, 1975).

Fodor’s theory is in contrary to behaviorism that explains human behaviours like causal relationships between stimuli as input and behavior as output. However, functionalism posits that such explanations will appeal to internal properties that mediate between inputs and outputs. In other words, functionalism explains the mental properties as causal relations where are not restricted only to input and output relations, but also include other properties that figure in the relevant empirical theories as well. Functionalism leads to multiple realisability by defining mental properties in regard to their causal roles and by allowing the various type of physical processes to fulfill these relations.